Don't magically import the microgrid module in examples

There is very little gain in making the import of the `microgrid` module
to happen automatically and it makes the `conftest.py` script not
generic enough to be shared by different projects (which is something
we eventually want to do).

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/conftest.py

@@ -125,8 +125,6 @@ def evaluate(self, example: Example) -> None | str:
         import_header.append(
             path_to_import_statement(Path(os.path.relpath(example.path)))
         )
-        # Add microgrid as default import
-        import_header.append("from frequenz.sdk import microgrid")
         imports_code = "\n".join(import_header)
 
         # Dedent the code example

src/frequenz/sdk/timeseries/_periodic_feature_extractor.py

@@ -79,6 +79,7 @@ class PeriodicFeatureExtractor:
 
     Example:
         ```python
+        from frequenz.sdk import microgrid
         from datetime import datetime, timedelta, timezone
 
         moving_window = MovingWindow(